Many technological fields may suggest examples of complex multilayer physical objects. Internal structure of such objects may be studied by analyzing available images of the objects. Examples of such analysis may be found in inspection of modern 3D semiconductor structures, as well as in geophysics, biology, medicine, in medical equipment technologies such as Computer Tomography, etc. The images to be analyzed may be obtained by various technologies utilized in the corresponding fields.
The present patent application will describe the mentioned technique using an example from the field of inspection of modern multilayer semiconductor structures.
The modern multilayer semiconductor structures of interest have arrived to such a scale of miniaturization (presently, up to nodes scale of about 7-10 nm) that they cannot be inspected with required accuracy and resolution by optical microscopes, since information provided by optical microscopes is a result of processing of visual images. For such modern structures, there is a theoretical option to apply complex, model-based analysis methods for processing data obtained by visual measurements.
An alternative, more practical option is to utilize technologies involving tools having resolution higher than that in optical microscopes.
Such tools, for example scanning electron microscopes (SEMs), are often used in inspection of semiconductor wafers. SEMs may be used to detect and classify defects in production of microelectronic devices, to provide sophisticated process control, etc. SEM images, however, contain a wealth of detail, which must be properly interpreted in order to identify the structures appearing in each image, to distinguish the structures from other features and to estimate their relative coordinates.
In order to proceed with the description, some comprehensive definitions have been introduced below, which are important for understanding the problem and the exemplary solutions which will be described below.
Three-dimensional integrated circuit (3D IC)—an integrated circuit manufactured by stacking silicon wafers and/or dies and interconnecting them vertically using through-silicon vias (TSVs) so that they behave as a single device. In the present description, we speak about 3D IC manufactured using fab processes by gradually depositing multiple layers (dies) one onto another. 3D IC is one preferred example of a Multilayer structure.
SEM—Scanning Electron Microscope used for exposing a 3D IC to a primary electron beam, collecting data on responsive electron beams or scattering electrons from multiple layers of the 3D IC and further reconstructing a combined SEM image of the multiple layers by applying computer processing to the collected data.
Available image or Given image—an image representing a real multilayer structure; SEM-image—an example of Available/Given image.
CD SEM—Critical Dimensions Scanning Electron Microscope, which is applicable in a wide range of nodes having dimensions from about 3000 nm to about 7 nm. CD-SEM delivers High Resolution, High Throughput, High Sensitivity and High Repeatability by utilizing more sophisticated electron optics and advanced image processing.
Expected image—image of one or more specific details/features/structures to be constructed and/or to be found in a specific layer of a multilayered structure. For example, the Expected image may be a design image, for example a CAD-image created by utilizing CAD (computer-aided design) tools for designing features of a specific layer. Expected image may be a design image, simulated so as to look closer to a real object after manufacturing thereof according to the design.
A combined Expected image—image obtained by combining Expected images of layers of the multilayer structure, taking into account visibility of the layers and elements thereof.
Overlay (OVL)—vector characterizing pattern-to-pattern alignment of one layer of a multilayer structure with respect to another layer thereof.
Modern silicon wafers are currently manufactured in a sequence of steps, each stage placing a pattern of material on the wafer; in this way transistors, contacts, etc., all made of different materials, are laid down. In order for the final device to function correctly, the separate patterns of the layers must be aligned correctly. Overlay control is control of the above-mentioned pattern-to-pattern alignment.
Registration—maximally possible alignment of two or more images by recognition of their mutual positioning. For example, registration may be performed by achieving maximal possible overlapping between the images. Various technologies of registration exist; a version of registration, customized for advanced semiconductor nodes will be proposed in the present description.
Segmentation of available image—labeling pixels of the available image to associate the pixels with different classes of features (objects, elements). The features may be located at different layers of the multilayer structure. Image segmentation is typically used to locate objects and boundaries (lines, curves, etc.) in images. More precisely, image segmentation is the process of assigning a label to every pixel in an image such that pixels with the same label share certain characteristics. For example, a specific label may indicate a specific layer of the structure.
Segmentation of SEM-image—labeling pixels of a SEM-image of a multilayer structure, in order to obtain a segmented image Segm(x,y) where similarly labeled pixels form segments. The Segmentation is a process of labeling pixels of the SEM image by assigning, to every pixel having coordinates(x,y), an index j being index of the layer to which this pixels belongs. The index j may accept values from the interval {1 . . . N} where N is a number of layers of the multilayer structure.
Maps of layers of an N-layer multilayer structure—is a set of N binary images each having the size of the given image.
Map of a specific layer is a binary image comprising features of that layer only, which are visible on the Given image (and consequently on the Segmented image), and which are represented on the Map by areas of pixels (segments) labeled with the label of that specific layer. It may be written down as follows:
Map (x, y){j}={1, if Segm (x,y)=j; 0 if Segm(x, y)≠j, 1<=j<=N, where N is a number of layers}.
The terms “area of pixels” and “segment” will be used intermittently in the description.
There is a long felt need in solutions for effective and accurate estimation of overlay in multilayer structures, for example such as modern micro miniaturized multilayer semiconductor structures.